Nina Munk’s new book, “The Idealist,” is about the well-known economist Jeffrey Sachs and his “quest to end poverty,” as the subtitle puts it. I know: That subtitle sounds like classic book-industry hyperbole, but, in this case, it’s not. That really is what Sachs has been trying to do. The question of whether or not he is succeeding is where things get tricky.

The quest began in 2005, when Sachs, who directs the Earth Institute at Columbia University, started an ambitious program called the Millennium Villages Project. He and his team chose a handful of sub-Saharan African villages, where they imposed a series of “interventions” in such areas as agriculture, health and education. The idea was that these villages would show Africa — and the world — how the continent could loosen the grip that extreme poverty had on so many of its people.

From the start, the Millennium Villages Project has been controversial. It has soaked up large sums of money — the original seed money was $120 million — which its critics believe could have been better used on more targeted, less grandiose forms of aid. Because Sachs, for years, refused — on ethical grounds, he said — to rigorously compare the results at his villages with villages that didn’t get the same kind of help, development experts complained that there was no way of knowing if the project was making a difference.

“There is zero scientific evidence that the Millennium Villages Project is meeting its goals,” says Michael Clemens, a senior fellow at the Center for Global Development and a leading Sachs critic. Sachs, for his part, insists that the Millennium Villages Project has been a roaring success, so much so that its interventions are now being imitated — “scaled up” — in nations like Uganda, with full government support.

It is not quite right to say that Nina Munk has sidestepped the dispute between Sachs and his critics. Mainly, though, she has looked at the Millennium Villages Project through a different lens. She has spent the last half-dozen years traveling back and forth to Africa, to see for herself how the Sachs experiment was unfolding. She focused in particular on two villages: Dertu, Kenya, and Ruhiira, Uganda.

“Jeff is a charismatic man, and I wanted to believe in him,” Munk told me recently. (She and I overlapped at Fortune for several years.) She is quick to give him credit where it is due: For instance, his passionate advocacy for free distribution of insecticide-coated bed nets, at a cost of hundreds of millions of dollars, is an important reason that the scourge of malaria is being reduced. But her reporting also caused her to become disillusioned, and humbled, by the difficulties that any Western aid effort is likely to encounter.

With almost every intervention, she documents the chasm that exists between the villagers and those running the project. At one point, the Millennium Villages Project persuades the farmers in Ruhiira to grow maize instead of their traditional crop, called matoke. “The results were fantastic,” she reports, a bumper crop.

Except there were no buyers for the maize, so most of it wound up being eaten by rats. In Dertu, Sachs’s staff decided it should set up a livestock market. It flopped. Efforts to convince villagers to start small businesses largely failed. The critical problem of getting clean water to the villages was enormously expensive.

Ultimately, reports Munk, Dertu was abandoned by the Millennium Villages Project while Ruhiira is today lauded as one of the project’s most successful villages. “There is no question the lives of people in Ruhiira have been improved,” Munk told me. “I’ve seen it.” But she is dubious about what that means — other than the fact that if you pump millions of dollars into an isolated African village, the villagers’ lives will be better. (Sachs’ defenders say that most of her reporting was done before the project really found its footing.)

And there’s the rub. Sachs has always aimed higher than helping a handful of villages. He wants to put his imprint on all of Africa. When I spoke to him recently, he claimed that is exactly what has been happening, thanks to more than $100 million in loans from the Islamic Development Bank. But when I looked at the press release announcing the “scale up” in Uganda, the money involved was less than $10 million — a pittance, really, and unlikely to transform the country.

That things in Africa are getting better is undeniable. Child mortality is down, as is the number of people living in extreme poverty. In his book, “Emerging Africa,” Steve Radalet, the former chief economist for the United States Agency for International Development, gives credit to such factors as more democratic governments, a new class of civil servants and businesspeople, and sounder economic policies. Sachs, on the other hand, wants us to believe that the main driver has been the Millennium Villages Project, which has shown the way.